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st: Repeated ANCOVA


From   VIVIAND Xavier <Xavier.Viviand@mail.ap-hm.fr>
To   statalist@hsphsun2.harvard.edu
Subject   st: Repeated ANCOVA
Date   Mon, 2 Dec 2002 09:01:32 +0100

Dear Statalist,


Before applying a repeated ANCOVA on my own data, I tried to run the example
proposed by Tabachnick and Fidell (Computer-assisted research design and
analysis, 2001) on page 417 (Table 8.13). The data set is a mixed ANCOVA
with a single covariate (Easy) measured once for each case. The
randomized-groups IV i type of music (labeled Music, 2 levels) and the
repeated-measures IV is time of day (4 levels, labeled T1 through T4).
The data are below :
Music Easy	T1	T2	T3	T4
1	6	8	7	5	3
1	6	9	7	5	1
1	4	8	5	4	1
1	3	6	6	4	2
2	6	2	4	8	10
2	7	6	5	9	10
2	3	2	4	6	10
2	5	3	5	6	9

The data set has been arranged for use in Stata:
music	easy	y	time	id
1	6	8	1	1
1	6	9	1	2
1	4	8	1	3
1	3	6	1	4
2	6	2	1	5
2	7	6	1	6
2	3	2	1	7
2	5	3	1	8
1	6	7	2	1
1	6	7	2	2
1	4	5	2	3
1	3	6	2	4
2	6	4	2	5
2	7	5	2	6
2	3	4	2	7
2	5	5	2	8
1	6	5	3	1
1	6	5	3	2
1	4	4	3	3
1	3	4	3	4
2	6	8	3	5
2	7	9	3	6
2	3	6	3	7
2	5	6	3	8
1	6	3	4	1
1	6	1	4	2
1	4	1	4	3
1	3	2	4	4
2	6	10	4	5
2	7	10	4	6
2	3	10	4	7
2	5	9	4	8

I entered the following command :
anova y easy music/id|music time time*music time*easy, rep(time) cont(easy)
I got the same same results than reported by Tabachnick and Fidell (using
SYSTAT GLM) except for the covariate :
	  	SS	df	MS	F	P
Stata  		.125     1        .125       0.18     0.6921
Tabachnick	11.327 1	11.327	15.96  0.01
and Fidell
How can this difference be explained ?
Secondly, I tried to test the homogeneity of regression slopes, i.e. the
interaction between the covariate Easy and the factor Music by
anova y easy easy*music music/id|music time time*music time*easy, rep(time)
cont(easy)
I got :
	  	SS	df	MS	F	P
	easy |        .125     1        .125       0.18     0.6921
easy*music |        0.00     0
        music |  .491276672     1  .491276672       0.69     0.4433
     id|music |   3.5483871     5  .709677419  
Why the df of easy*music is 0 ?

Sincerely yours, 

Xavier VIVIAND 





	
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